Perfluorinated or polyfluorinated compounds refer to a type of compounds wherein all or part of hydrogens (H) connecting to the carbons (C) are substituted by fluorine (F). The most representative perfluorinated compounds include perfluorooctane sulfonic acid and its salts (PFOS) and perfluorooctanoic acid and its salts (PFOA). They have been widely used in industrial products and consumer goods since 1950s: PFOS is an outstanding surfactant, during the past fifty years, it was extensively used as textile or leather finishing agent, foam extinguishing agent, oil drilling additive and fog inhibitor in electroplating; PFOA is an important raw material for the manufacture of high-effective fluoropolymers, which is used in the surface coating of aviation devices, electronic components and kitchen wares.
However, recent researches show that the perfluorinated or perfluorinated compounds, with PFOS and PFOA as their representatives, present such disadvantages as long-term persistence, high bioaccumulation, strong biotoxicity and capability for long-distance migration. They hardly degrade in the natural environment. Since 1990s, PFOS and PFOA have been frequently detected out in surface water, ground water, sediments, bodies of both animals and human beings. These facts have caused great concerns throughout the world and many actions have been taken to deal with the PFOS- and PFOA-related pollution. In 2006, the European Union issued Restrictions on the Marketing and Use of Perfluorooctane Sulfonate. In the same year, the United States Environmental Protection Agency (EPA) also launched a self-initiated plan requiring the related enterprises to reduce 95% of PFOA discharge by 2010 and to realize zero discharge of PFOA by 2015. In May of 2009, the Fourth Session of the Conference of Parties listed PFOS in Annex A of Stockholm Convention on Persistent Organic Pollutants (POPs Treaty).
With PFOS being listed in the POPs Treaty and banned internationally, some other perfluorinated or polyfluorinated compounds appeared on the market as substitutes to PFOS and PFOA. These compounds include perfluorinated ones such as perfluorobutane sulfonic acid and its salts (PFBS) (typical CAS numbers: 375-73-5, 29420-49-3), perfluorohexane sulfonic acid and its salts (PFHxS), (typical CAS numbers: 355-46-4, 3871-99-6) and perfluoroalkyl ether potassium sulfonate (trade name: F-53B) (typical CAS number: 73606-19-6), and polyfluorinated ones such as 6:2 fluorotelomer sulfonic acid and its salts (6:2 FTS) (typical CAS numbers: 27619-97-2, 425670-75-3). Except presenting lower bioaccumulation, these new compounds do not show significant improvements in respect of degradation or persistence in comparison with PFOA and PFOS.
In view of this situation, finding appropriate methods to treat solid wastes containing perfluorinated or polyfluorinated compounds are of great practical importance. Currently, the commonest way to degrade solid wastes containing perfluorinated compounds is high-temperature incineration, which not only requires strict reaction conditions and reaction equipments, but also is likely to produce hydrogen fluoride, an acidic corrosive gas, and even the byproduct dioxin. Therefore, it is very urgent to develop non-incineration technologies for treating solid wastes containing perfluorinated or polyfluorinated compounds.
The mechanochemical method is to degrade the pollutant by mixing the solid pollutant and the reaction reagent together into a high-energy ball milling reactor and starting chemical reaction between them by means of mechanic force. Most of existing documents, including patents and articles, on the mechanochemical method involves the treatment of chlorinated persistent organic pollutants, and currently there is only one document [Masayuki Shintani, Yuta Naito, Shingo Yamada, et, al. “Degradation of Perfluorooctansulfonate (PFOS) and Perfluorooctanoic Acid (PFOA) by Mechanochemical Treatment” Kagaku Kogaku Ronbunshu (Journal of Chemical Engineering of Japan) 34.5 (2008): 539-544.] disclosed a mechanochemical method for treating fluorinated persistent organic pollutants (such as PFOS or PFOA), the carbon-fluorine bond of which contains higher bond energy. In this essay, the common calcium oxide was mixed with PFOS and PFOA separately, then the mixture was put into a planetary ball mill and milled at the rotation rate of 700 rpm; after 3 hours (for PFOS) and 18 hours (for PFOA) of reaction, both PFOS and PFOA are almost completely decomposed. However, the detected amount of inorganic fluorine is negligible (lower than 1% of theoretical yield), and in the case of PFOS, the highest amount of detected sulfate ion is lower than 50% of theoretical yield. The result of this disclosed document suggests that on the one hand, the mechanochemical method is technically feasible for treatment of PFOS and PFOA, and on the other, the adoption of calcium oxide as the reaction reagent presents considerable limitations.
In practical process of wastes treatment, we not only expect transformation of the target material, but also hope that the fluorine therein can effectively turn inorganic, which is an extremely important sign of defluorination and detoxication of PFOS and PFOA. That is to say, the existing method should be further improved. In addition, in view of the fact that most of perfluorinated or polyfluorinated substitutes still present high indegradability and strong persistence, it is necessary to find out suitable treatment methods for high-effectively degrading these new compounds.